Introduce painted-device-scale-factor and use it when --enable-use-zoom-for-dsf is specified.

third_party/WebKit/Source/web/WebViewImpl.cpp

Index: third_party/WebKit/Source/web/WebViewImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebViewImpl.cpp b/third_party/WebKit/Source/web/WebViewImpl.cpp
index 2ff804ebb0cef02fe89bcd33c78e41db630d25b3..17c1bcb6fd2f29748fa24f248b03bdc64562e861 100644
--- a/third_party/WebKit/Source/web/WebViewImpl.cpp
+++ b/third_party/WebKit/Source/web/WebViewImpl.cpp
@@ -412,6 +412,8 @@ WebViewImpl::WebViewImpl(WebViewClient* client)
     , m_zoomLevel(0)
     , m_minimumZoomLevel(zoomFactorToZoomLevel(minTextSizeMultiplier))
     , m_maximumZoomLevel(zoomFactorToZoomLevel(maxTextSizeMultiplier))
+    , m_zoomFactorForDeviceScaleFactor(1.f)
+    , m_useZoomForDeviceScaleFactor(false)
     , m_maximumLegibleScale(1)
     , m_doubleTapZoomPageScaleFactor(0)
     , m_doubleTapZoomPending(false)
@@ -2958,6 +2960,7 @@ double WebViewImpl::setZoomLevel(double zoomLevel)
     LocalFrame* frame = mainFrameImpl()->frame();
     if (!WebLocalFrameImpl::pluginContainerFromFrame(frame)) {
         float zoomFactor = m_zoomFactorOverride ? m_zoomFactorOverride : static_cast<float>(zoomLevelToZoomFactor(m_zoomLevel));
+        zoomFactor *= m_zoomFactorForDeviceScaleFactor;
         frame->setPageZoomFactor(zoomFactor);
     }
 
@@ -3094,13 +3097,26 @@ float WebViewImpl::deviceScaleFactor() const
 
 void WebViewImpl::setDeviceScaleFactor(float scaleFactor)
 {
-    if (!page())
-        return;
+    if (m_useZoomForDeviceScaleFactor) {
+        if (!m_layerTreeView || m_zoomFactorForDeviceScaleFactor == scaleFactor)
+            return;
+        m_zoomFactorForDeviceScaleFactor = scaleFactor;
+        setZoomLevel(m_zoomLevel);
+    } else {
+        if (!page())
+            return;
 
-    page()->setDeviceScaleFactor(scaleFactor);
+        page()->setDeviceScaleFactor(scaleFactor);
 
-    if (m_layerTreeView)
-        updateLayerTreeDeviceScaleFactor();
+        if (!m_layerTreeView)
+            return;
+    }
+    updateLayerTreeDeviceScaleFactor();
+}
+
+void WebViewImpl::enableUseZoomForDeviceScaleFactor()
+{
+    m_useZoomForDeviceScaleFactor = true;
 }
 
 void WebViewImpl::setDeviceColorProfile(const WebVector<char>& colorProfile)